Single photons emitted by each of the
ions are routed through optical fibers to a beamsplitter in which any arriving
photon has a 50-50 chance of passing through or reflecting off. Before hitting
the splitter, each photon is in a superposition of red and blue colors. When
photons emerge from different sides of the beamsplitter, however, they are
forced into opposite states -- red/blue or blue/red -- at random. In this case,
each detector will record a photon at the same time -- one red and one blue. But
it is impossible to know which ion produced which photon. A blue photon in the
left detector, for example, could have come from Ion A and been reflected at the
splitter. Or it could have come from Ion B and passed directly through. This
fundamental uncertainty projects the ions into an entangled state, a condition
immediately signaled by the simultaneous detection of two photons.
(Credit: Image courtesy of University of Maryland)

ScienceDaily (Jan. 23, 2009) — For the
first time, scientists have successfully teleported information between two
separate atoms in unconnected enclosures a meter apart – a significant milestone
in the global quest for practical quantum information processing.

Teleportation may be nature's most mysterious form of transport:
Quantum information, such as the spin of
a particle or the polarization of a photon, is transferred from one place to
another, without traveling through any physical medium. It has previously
been achieved between photons over very large distances, between photons and
ensembles of atoms, and between two nearby atoms through the intermediary action
of a third. None of those, however, provides a feasible means of holding and
managing quantum information over long distances.

Now a team from the Joint Quantum Institute (JQI) at the University of Maryland
(UMD) and the University of Michigan has
succeeded in teleporting a quantum state directly from one atom to another over
a substantial distance. That capability is necessary for workable quantum
information systems because they will require memory storage at both the sending
and receiving ends of the transmission.

In the Jan. 23 issue of the journal Science, the scientists report that, by
using their protocol, atom-to-atom teleported information can be recovered with
perfect accuracy about 90% of the time – and that figure can be improved.

"Our system has the potential to form
the basis for a large-scale 'quantum repeater' that can network quantum memories
over vast distances," says group leader Christopher Monroe of JQI and
UMD. "Moreover, our methods can be used in conjunction with quantum bit
operations to create a key component needed for quantum computation." A quantum
computer could perform certain tasks, such as encryption-related calculations
and searches of giant databases, considerably faster than conventional machines.
The effort to devise a working model is a matter of intense interest worldwide.

Teleportation works because of a
remarkable quantum phenomenon, called "entanglement," which only occurs on the
atomic and subatomic scale. Once two objects are put in an entangled state,
their properties are inextricably entwined. Although those properties are
inherently unknowable until a measurement is made, measuring either one of the
objects instantly determines the characteristics of the other, no matter how far
apart they are.

The JQI team set out to entangle the quantum states of two individual ytterbium
ions so that information embodied in the condition of one could be teleported to
the other. Each ion was isolated in a separate high-vacuum trap, suspended in an
invisible cage of electromagnetic fields and surrounded by metal electrodes.
[See illustrations.] The researchers identified two readily discernible ground
(lowest energy) states of the ions that would serve as the alternative "bit"
values of an atomic quantum bit, or qubit.

Conventional electronic bits (short for binary digits), such as those in a
personal computer, are always in one of two states: off or on, 0 or 1, high or
low voltage, etc. Quantum bits, however,
can be in some combination, called a "superposition," of both states at the same
time, like a coin that is simultaneously heads and tails – until a measurement
is made. It is this phenomenon that gives quantum computation its extraordinary
power.

At the start of the experimental
process, each ion (designated A and B) is initialized in a given ground state.
Then ion A is irradiated with a specially tailored microwave burst from one of
its cage electrodes, placing the ion in some desired superposition of the two
qubit states – in effect writing into memory the information to be teleported.

Immediately thereafter, both ions are excited by a picosecond (one trillionth of
a second) laser pulse. The pulse duration is so short that each ion emits
only a single photon as it sheds the energy gained from the laser pulse and
falls back to one or the other of the two qubit ground states. Depending on
which one it falls into, each ion emits a photon whose color (designated red and
blue) is perfectly correlated with the two atomic qubit states. It is this
entanglement between each atomic qubit and its photon that will eventually allow
the atoms themselves to become entangled.

The emitted photons are captured by lenses, routed to separate strands of
fiber-optic cable, and carried into opposite sides of a 50-50 beamsplitter where
it is equally probable for either photon to pass straight through the splitter
or to be reflected. On either side of the beamsplitter output are detectors that
can record the arrival of a single photon.

Before reaching the beamsplitter, each photon is in a superposition of states.
After encountering the beamsplitter, four color combinations are possible:
blue-blue, red-red, blue-red and red-blue. In nearly all of those variations,
the photons cancel each other out on one side and both end up in the same
detector on the other side. But there is one – and only one – combination in
which both detectors will record a photon at exactly the same time.

In that case, however, it is physically impossible to tell which ion produced
which photon because it cannot be known whether the photon arriving at a
detector passed through the beamsplitter or was reflected by it.

Thanks to the peculiar laws of quantum mechanics, that inherent uncertainty
projects the ions into an entangled state. That is, each ion is in a correlated
superposition of the two possible qubit states. The simultaneous detection of
photons at the detectors does not occur often, so the laser stimulus and photon
emission process has to be repeated many thousands of times per second. But when
a photon appears in each detector, it is an unambiguous signature of
entanglement between the ions.

When an entangled condition is
identified, the scientists immediately take a measurement of ion A. The act of
measurement forces it out of superposition and into a definite condition: one of
the two qubit states. But because ion A's state is irreversibly tied to ion B's,
the measurement of A also forces B into a complementary state. Depending on
which state ion A is found in, the researchers now know precisely what kind of
microwave pulse to apply to ion B in order to recover the exact information that
had originally been stored in ion A. Doing so results in the accurate
teleportation of the information.

What distinguishes this outcome as
teleportation, rather than any other form of communication, is that no
information pertaining to the original memory actually passes between ion A and
ion B. Instead, the information disappears when ion A is measured and reappears
when the microwave pulse is applied to ion B.

"One particularly attractive aspect of our method is that it combines the unique
advantages of both photons and atoms," says Monroe. "Photons are ideal for
transferring information fast over long distances, whereas atoms offer a
valuable medium for long-lived quantum memory. The combination represents an
attractive architecture for a 'quantum repeater,' that would allow quantum
information to be communicated over much larger distances than can be done with
just photons. Also, the teleportation of quantum information in this way could
form the basis of a new type of quantum internet that could outperform any
conventional type of classical network for certain tasks."

The Joint Quantum Institute is a partnership effort between the National
Institute of Standards and Technology and UMD, with additional support from the
Laboratory for Physical Science. The work reported in Science was supported by
the Intelligence Advanced Research Project Activity program under U.S. Army
Research Office contract, the National Science Foundation (NSF) Physics at the
Information Frontier Program, and the NSF Physics Frontier Center at JQI.